There is no admission that the background art disclosed in this section legally constitutes prior art.
New energy systems are required that can operate on fossil fuels and yet generate few greenhouse gases and polluting emissions. This is becoming an increasing concern since the world's energy infrastructure essentially dictates the continued (and increased) use of fossil fuels for the next several years and maybe decades. The use of different types of resources, such as coal and natural gas, can still provide especially attractive energy systems. In order to be practical, however, the cost of these new systems must be comparable to that of current energy production technology. This presents an ongoing concern since, in major markets, the efficiency and the environmental performance of such energy systems are not likely to warrant premium prices.
However, currently used logistic fuels, such as coal, diesel and jet fuel, while still an attractive option for power generation compared to the traditional hydrocarbons (such as gasoline), often contain unacceptably high levels of sulfur. For example, while diesel-based logistic fuel (for trucks and locomotives) is the main artery of transportation in the continental US as well as across the globe, diesel fuel is invariably sulfur-laden which can have dramatic effect on the overall fuel economy. Its combustion is attended by emission of sulfur containing species (SOx mainly).
The presence of sulfur and the gradual build-up and accumulation of sulfur-bearing compounds in the exhaust stream has its toll on the catalytic converters in the vehicles as well. Over time, the catalyst in the catalytic converter is poisoned by the organosulfurs that are invariably present in the diesel fuel. This necessitates a periodic purging and cleaning of the catalyst which is a high-temperature energy-intensive oxidative process (done in a rich air-to-fuel ratio environment) that affects the fuel economy of the vehicle adversely. In the case of other logistic fuels (such as jet and aviation fuels as well as coal) sulfur also causes deleterious effects to the reforming catalysts which are severely poisoned and deactivated.
Because of these problems, one attempt to provide a different energy system includes the development of fuel cell technology. Fuel cells are environmentally clean, quiet, and highly efficient devices for generating electricity and heat from natural gas, biomass, and other fuels.
Also, the fuel cells themselves are vastly different from other power sources. To those skilled in the art, a fuel cell is an electrochemical device that converts the chemical energy of a fuel directly into electrical energy and the associated heat without combustion or moving parts. As such, fuel cells have emerged in the past decade as one of the most promising new technologies for meeting the world's increasing energy needs.
Fuel cells continuously convert chemical energy into electric energy for as long as fuel and oxidant are supplied. Different categories of fuel cells are known, including proton exchange membrane fuel cells (PEMFCs) and solid oxide fuel cells (SOFCs) which are both fueled by hydrogen.
In general, a solid oxide fuel cell (SOFC) operates by receiving reformate (also referred to as a hydrogen-rich gas stream) at an anode inlet; and air or an oxygen containing gas stream at a cathode inlet. A voltage is generated across the anode/electrolyte/cathode assembly in an open-circuit mode (under zero electrical load). In the presence of an applied load in the form of imposed current, charges are driven in the external circuit and electrical power (=current×voltage) is generated. The SOFC generally has a dense ceramic membrane, permeable only to either oxygen or hydrogen ions; called an oxygen ion or a proton (hydrogen ion) conductor, respectively. With solid electrolyte membranes that are comprised of an oxygen ion conductor, the molecular oxygen at the porous cathode ionizes by picking up electrons from the external circuit, moves through and across the electrolyte, and reacts with hydrogen (or other fuel components) at the anode/electrolyte interface, thereby forming water (or other products) and releasing electrons to the circuit, thus completing the circuit.
In SOFCs that use logistic sources of fuel, the hydrogen-rich fuel stream can contain small fractions of carbon monoxide, carbon dioxide, low hydrocarbons such as methane or ethane, with water vapors and/or some nitrogen as diluents, along with undesirable sulfur-laden components.
In addition, SOFCs can also be fueled by other fuels such as carbon monoxide, natural gas and other hydrocarbons. The primary advantages of fuel cell power generation include increased efficiency, lower weight, smaller size, less air pollution, and reduced noise.
Fuel cells are being considered for use in many different applications. For example, they may be used to power automobiles such as passenger cars and light-duty trucks, and naval vessels including surface ships and submarines. NASA envisions employing SOFCs running on jet fuel reformate for its Uninhabited Aerial Vehicle (UAV) and Low Emission Alternative Power (LEAP) missions, as well as for transatlantic and intercontinental commercial airline flights.
The U.S. military is also considering the use of fuel cells that are fueled by jet fuel reformate where the jet fuel is subjected to a reforming process in a fuel processor to produce a hydrogen-rich reformate. However, depending on the source and kind, jet fuels are invariably sulfur-laden. When sulfur is present in any fuel that is used in a fuel cell, the sulfur poisons the fuel cell anode and thereby degrades the performance of the fuel cell. Also, the sulfur present in the fuel poisons the reforming catalyst that is used in the reforming process.
Currently, the fuel reformer uses a catalytic support which is an inert matrix of alumina, silica, magnesia or zirconia. The catalytic support is impregnated with a noble metal catalyst, for example, such as Pt, Pd, Rh and/or Au, or a non-precious metal catalyst such as Ni or Cu.
U.S. Pat. No. 6,713,040 to Ahmed et al., assigned to Argonne National Laboratory, discloses a sulfur-tolerant reforming catalyst consisting of a transition metal supported on a doped ceria. The catalyst is said to be useful for reforming a wide variety of different fuels, including jet fuels. After a reforming process, the reformate gas passes through a sulfur removal zone which includes a sulfur removal agent such as zinc oxide.
Ming et al., Catalysis Today, 77 (2002) 51, disclose that Innovatek has developed a sulfur-tolerant (up to 100 ppm sulfur) reforming catalyst for conversion of diesel fuel that operates at a steam to carbon ratio of 3.6 for 220 hours with no deactivation.
Recently, the Pacific Northwest National Laboratory (PNNL) demonstrated such an operation to run a 5-kW SOFC unit on JP-8, a fuel commonly used in military operations (Alex Hutchinson, “Portable fuel cell runs on military jet fuel to power diesel trucks,” Popular Mechanics, Dec. 12, 2007; (see web site: fuelcellsworks.com/Supppage 8217). Due to their proprietary nature, not many details of the composition of the desulfurizer and/or the reforming catalyst used by PNNL are known. It is believed, however, that the catalytic hydrodesulfurization process developed by PNNL removes sulfur from the JP-8 fuel using syngas as the co-reactant in place of hydrogen. The gas phase operation of the process allows for a significant increase in throughput and a decrease in operating pressure compared with conventional technology. Further, it is believed that the process does not require consumables or periodic regeneration. However, to those skilled in the art, it is evident that the PNNL design though attractive, necessitates the operation of two units independently and hence, is likely to incur higher cost, difficulty in system integration as well as the lack of efficient thermal management.
The current leading fuel cell technology under consideration for transportation and distributed residential power applications is based on a polymer electrolyte membrane fuel cell (PEMFC). This type of fuel cell operates at low temperatures (generally less than about 100° C.). By operating on hydrogen as the energy carrier, very high power conversion efficiencies are possible with the PEMFC, and only water is produced as a byproduct. In reality though, a hydrogen infrastructure that will support large markets is years or even decades away.
Therefore, in the PEMFC, hydrocarbon (i.e., fossil) fuels must first be converted (or reformed) into a hydrogen-rich gas with little or no carbon monoxide or other poisons (e.g., sulfur-bearing species and ammonia). This calls for purification steps with several catalytic stages (such as, low and high temperature water-gas-shift reaction, sulfur removal and ammonia scrubbing), thereby increasing PEMFCs overall complexity and imposing a premium price tag on the PEMFC due to the requirements of very high purity hydrogen and the presence of noble metals in the electrodes.
Fuel cells are primarily characterized by their electrolyte material and, as the name implies, the solid oxide fuel cells (SOFCs) have a solid ceramic oxide electrolyte. The SOFCs generally operate at high temperatures (about 800 to about 1000° C.). Power in the SOFC is generated in multilayer ceramic cells each of which comprises a porous anode layer, a dense electrolyte layer, and a porous cathode layer. Individual cells are connected to each other via an interconnect, thereby making a stack. Power generation in the SOFC involves conversion of oxygen molecules (from air) to oxygen ions at the cathode, conductance of oxygen ions through the electrolyte, and reaction of these oxygen ions with fuel at the anode to form water and carbon dioxide.
For example, SOFC systems operating with natural gas as a fuel can achieve power generation efficiencies in the range of 40 to 45 percent. Hybrid systems, combining solid oxide fuel cells and gas turbines, can achieve efficiencies of up to 70 percent. In another example, Siemens-Westinghouse has been developing SOFC technology for stationary, megawatt-scale power systems operating on natural gas. Their field tests have demonstrated exceptional reliability, with degradation rates less than 0.1 percent per decade over thousands of hours of operation. However, the Siemens-Westinghouse SOFC systems utilize external reformers and are relatively expensive, with projected installed costs of $1500/kW.
What is needed is a method to reduce the costs of SOFC power generation. Such systems would provide attractive options for smaller-scale (5˜20 kW) power generation applications within various residential, transportation, industrial, and military market segments. Currently, in the industry, the Solid State Energy Conversion Alliance (SECA)'s goal is to facilitate these cost reduction efforts, with a cost target of $400/kW for 5-kW systems.
One challenge in providing the needed hydrogen-rich fuel stream derived from the logistic fuels described herein for power generation via SOFC stacks, is the presence of significant amounts of sulfur (mainly as organosulfurs) in these fuels. For example, the sulfur level in liquid jet fuels varies between 0.3 to 1%, while that in gasified coal could vary between 2.3 and 4.5%, depending upon the quality of the bituminous coal. This presents a particular challenge since the desulfurization of syngas and the recovery of sulfur are highly energy intensive and remain the major hurdles to be overcome in improving the economics of producing sulfur-free hydrogen-rich streams from logistic fuel sources.
For example, currently used hydrodesulfurization systems, such as ZnO or alumina-supported molybdenum sulfide promoted by nickel or cobalt, must operate in the range of 320-440° C. under conditions of very high H2 pressure (˜15-225 atm). The requirement of high H2 pressure in these hydrodesulfurization systems causes both operational and occupational safety issues in the vicinity of an SOFC-based powder generation system.
In addition, the ZnO-based sorption technology in many respects, is an unsatisfactory technology, having such problems as the volatilization of elemental zinc and the formation of ZnSO4 with concomitant volume expansion during regeneration. Moreover, the generation of a SOFC-quality fuel from logistic fuels necessitates a separate reforming step in addition to the desulfurization step, thus increasing the system complexity and cost.
Further, while ceria-based systems offer the possibility of gasified coal desulfurization, the direct production of elemental sulfur during sorbent regeneration necessitates the supply of SO2 externally which, in turn, substantially complicates the system and greatly increases its costs. Moreover, this art requires the use of pressures in the range of about ˜5 atmospheres and higher.
In order to overcome the shortcoming of the existing desulfurization technologies, there is a need for sorbent matrices that would be thermally and microstructurally stable and sulfur-active at high temperatures, and that would not require regeneration.
Furthermore, there is a need for robust sulfur-tolerant steam-reforming catalysts.
It would be particularly advantageous if a compact system were developed that would meet both these needs and, would generate high quality H2-rich fuel stream from logistic fuels for SOFCs.
It would also be advantageous if such a ready-to-use high quality H2-rich fuel stream could be derived in a single-reactor module. There is a particular need for a simplified and efficient system that produces SOFC-quality feed in one step.
It would also be advantageous if a system could be developed that did not require a hydrodesulfurization step that uses hydrogen for desulfurization.
It would also be advantageous to provide a system where desulfurization, reforming and power generation, all occur at temperatures less than about 1000° C., thereby obviating the need for conditioning the syngas stream and enhancing the overall efficiency of the entire unit, from desulfurization to reforming to power generation.